The reduction of the tryptophan tryptophylquinone [TTQ] prosthetic group of aromatic amine dehydrogenase by dopamine exhibits a deuterium kinetic isotope effect of 8.6-11.7. This effect is partially suppressed in the steady-state. These data support the proposed mechanism for the reductive half-reaction of AADH in which TTQ reduction is linked to proton abstraction from a covalent enzyme-substrate intermediate and in which release of the aldehyde product is partially rate-limiting for the overall reaction. The magnitude of this kinetic isotope effect appears to exceed the semiclassical limit for a proton abstraction. It is also nearly identical to values for the similar reactions catalyzed by methylamine dehydrogenase, which also possesses TTQ, and plasma amine oxidase, which possesses the topaquinone cofactor. These data suggest that these three quinoproteins share a very similar and possible unique mechanism for catalyzing the oxidative deamination of primary amines.